Exponential conversion circuit

Abstract

A first voltage conversion circuit converts first and second reference input voltages into first and second differential output voltage. A second voltage conversion circuit converts the first reference input voltage and a control input voltage into a third differential output voltage. The third differential output voltage is inputted to an exponential conversion element. The first and second differential output voltages are inputted to an active impedance bridge. The active impedance bridge outputs a gain control voltage of the first and second voltage conversion circuits. A balanced condition of the active impedance bridge determines the exponential conversion characteristic of the output current to the control input voltage of the exponential conversion element.

1. An exponential conversion circuit, comprising:
 a first voltage conversion circuit configured to convert first and second reference input voltages to first and second differential output voltages on the basis of a gain control signal;  
 a second voltage conversion circuit configured to convert the first reference input voltage and a first control input voltage to a third differential output voltage on the basis of the gain control signal;  
 an exponential conversion element configured to produce an output current which varies exponentially according to the third differential output voltage; and  
 an impedance bridge including a first, a second, a third and a fourth transconductance elements and an amplifier configured to produce the gain control signal to provide to the first and second voltage conversion circuit according to the first and the second differential output voltages, the first transconductance element receiving the first differential output voltage, the second transconductance element receiving the second differential output voltage, the third transconductance element receiving the output voltage of the amplifier and the fourth transconductance element receiving a second control input voltage, and the amplifier producing the output voltage according to the difference between the output current of the first transconductance element and the output current of the second transconductance element.  
 
   
   
     2. The exponential conversion circuit according to  claim 1 , wherein the first) the second, the third and the fourth transconductance elements have a transconductance gm 1 , gm 2 , gm 3 , and gm 4  respectively, and an equation gm 2 /gm 1 =gm 4 /gm 3  is realized as a balanced condition. 
   
   
     3. The exponential conversion circuit according to  claim 1 , wherein the second control input voltage determines an inclination of the exponential conversion characteristic. 
   
   
     4. The exponential conversion circuit according to  claim 2 , wherein the second control input voltage determines an inclination of the exponential conversion characteristic. 
   
   
     5. The exponential conversion circuit according to  claim 1 , wherein the exponential conversion element includes a MOS transistor which operates in a weak inversion area. 
   
   
     6. The exponential conversion circuit according to  claim 2 , wherein the exponential conversion element includes a MOS transistor which operates in a weak inversion area. 
   
   
     7. The exponential conversion circuit according to  claim 3 , wherein the exponential conversion element includes a MOS transistor which operates in a weak inversion area. 
   
   
     8. The exponential conversion circuit according to  claim 4 , wherein the exponential conversion element includes a MOS transistor which operates in a weak inversion area. 
   
   
     9. The exponential conversion circuit according to  claim 1 , wherein the first, the second, the third and the fourth transconductance elements respectively includes a MOS transistor which operates in a weak inversion area. 
   
   
     10. The exponential conversion circuit according to  claim 2 , wherein the first, the second, the third and the fourth transconductance elements respectively includes a MOS transistor which operates in a weak inversion area. 
   
   
     11. The exponential conversion circuit according to  claim 3 , wherein the first, the second, the third and the fourth transconductance elements respectively includes a MOS transistor which operates in a weak inversion area. 
   
   
     12. The exponential conversion circuit according to  claim 4 , wherein the first, the second, the third and the fourth transconductance elements respectively includes a MOS transistor which operates in a weak inversion area. 
   
   
     13. The exponential conversion circuit according to  claim 5 , wherein the first, the second, the third and the fourth transconductance elements respectively includes a MOS transistor which operates in a weak inversion area. 
   
   
     14. The exponential conversion circuit according to  claim 6 , wherein the first, the second, the third and the fourth transconductance elements respectively includes a MOS transistor which operates in a weak inversion area. 
   
   
     15. The exponential conversion circuit according to  claim 7 , wherein the first, the second, the third and the fourth transconductance elements respectively includes a MOS transistor which operates in a weak inversion area. 
   
   
     16. The exponential conversion circuit according to  claim 8 , wherein the first, the second, the third and the fourth transconductance elements respectively includes a MOS transistor which operates in a weak inversion area.